1. Technical Field of the Invention
The present invention relates to thermally insulated double-walled synthetic resin containers and their lids, as well as manufacturing methods therefor, used as heat preserving containers such as cooler boxes, vacuum flasks, heat preserving lunchboxes, rice bowls, soup bowls and cups.
2. Background Art
One type of thermally insulated double-walled synthetic resin container which has been conventionally proposed is one wherein a double-walled container formed from a synthetic resin inner container and outer container are filled with a gas which has a lower thermal conductivity than air in the space layer. This thermally insulated double-walled synthetic resin container has a plated metallic film formed on the surface of the space layer side of the double-wall for the purposes of increasing the gas barrier capabilities.
While polycarbonate resins are usually used for the inner and outer containers for their hot-water-resistance, it is not possible to form a plating film directly onto the outer surface of an inner container or the inner surface of an outer container formed from polycarbonate resins, so that a coating which includes resins that are capable of being plated are coated before applying the plating film.
However, since polycarbonate resins have low chemical resistance to organic solvents, bleaches, detergents and the like, even if a coating is applied to the surfaces on which plating films are to be formed, there is the risk that the solvent for the coating may cause cracks (solvent cracks) to form in the molded article, in which case the gas barrier capabilities of the thermally insulated double-walled synthetic resin container would be lost. Additionally, when sealing with a sealing panel by using adhesives during the post-processing wherein a low thermal conductivity gas is sealed within the space portion of the double-walled container, there is the risk of the solvent contained in the adhesive forming cracks near the sealed portion or in the sealing panel. Furthermore, cracks may be formed during actual use by the contents such as foods and beverages, bleaches, or various detergents.
Additionally, the gas barrier capabilities of polycarbonate resins themselves are not very good, so that a high-precision plating film must be applied in order to confer gas barrier capabilities, thereby increasing the manufacturing cost.
Additionally, as for thermally insulated containers such as cooler boxes, a double-walled container formed from a synthetic resin inner container and outer container having a gas with a lower thermal conductivity than air sealed into the thermally insulating space layer has been proposed. Since the thermally insulating space layer inside the thermally insulated double-walled synthetic resin container is exchanged and sealed with low thermal conductivity gas, a hole portion for exchange and sealing is formed at a portion of the outer container or the inner container.
This hole portion is provided at a position which is not externally visible. Additionally, after joining the inner container and the outer container, low thermal conductivity gas is exchanged into the space formed by the inner and outer containers by means of the hole portion and sealed by means of a sealing panel; the hole portion is usually provided in the center of the bottom of the outer container for ease of performing such procedures.
However, since a metallic mold must be made in order to put a hole portion in the bottom center of the outer container when the outer container is molded, the position of the gate on the outer container for extrusion molding the metallic mold must be provided at a place aside from the bottom center of the outer container. For this reason, the flow of synthetic resins into the molding space of the metallic mold for forming the outer container so as to be right/left symmetric with respect to the center becomes non-uniform with respect to the circumferential wall end portion of the outer container, so that the outer container molded article often has imperfect welds, thereby increasing the likelihood of insufficient strength or defects in appearance such as decentralization, warping and the like, and further raising the costs due to increased defectiveness.
Additionally, if the gate for mold extrusion is positioned at the bottom center of the outer container, a hole opening process for the gate cutting portion is required after molding. This hole opening process results in cost increases during molding because precise positioning is required due to the small hole diameter of approximately 1 mm. As a result, the outer container cannot be manufactured cheaply.
Additionally, while a hole portion is formed in the bottom center of an outer container for the gas filling procedure when a thermally insulating effect is to be gained by filling the space between the inner and outer containers of a thermally insulated double-walled synthetic resin container, there is a need to protect this hole portion. For this reason, a concave portion having the hole portion at the center is provided and the sealing is performed by fitting a sealing panel into this concave portion, which presents a problem in that the area around the concave portion is made thinner due to the formation of the concave portion so as to weaken the strength.
Additionally, as thermally insulated food containers which have been conventionally proposed, double-walled containers formed by joining a synthetic resin inner container and outer container with the thermally insulating space layer being air-insulated, as well as those provided with insulating materials such as styrofoam, urethane and the like, are known. These conventional thermally insulated double-walled synthetic resin containers do not offer sufficient heat preserving and cold preserving performance, so that they are not satisfactory for actual use. Additionally, the thickness of the thermally insulating space layer can be increased in order to improve the thermal insulation performance, but in actual practice, the effective capacity proportion between the internal storage capacity and the apparent volume of the double-walled container when turned over onto a flat surface is made worse, so that the product value is extremely degraded.
Additionally, thermally-insulated metallic vacuum containers have been proposed as a solution thereto.
However, food containers usually have large mouths, so that they have significant thermal conductivity loss from their mouths; especially in the case of metallic containers, the thermal conductivity loss is large in comparison to those using synthetic resins, so that they have low thermal insulation capabilities. There is also the simultaneous danger that the mouth portion can become hot. Furthermore, metallic thermally insulated containers are undesirable for various other reasons such as being heavy and expensive.
Additionally, conventional food containers which allow heat preservation are formed from a container body for putting in food and a lid, with the thermal insulator layer inside the container being filled with insulators such as styrofoam and urethane, or with thermal insulation due to air. Some such container bodies are leaved with aluminum in the thermal insulator layer in order to suppress radiative heat emissions.
Additionally, some of the lids attached to the above containers have a double-layered structure with a handle portion on the top portion of the lid, but these stress the heat preserving properties of the container body; there are no proposals which take into consideration the storage capabilities and heat preserving properties of the lid.
The lids of the above-mentioned containers have less heat preserving capabilities in comparison to the container bodies, so that the heat from the containers easily escapes through their lids and the appropriate temperature is lost by the time the contents are eaten. Additionally, if the container is stuffed with a lot of food, moisture from the food inside the container often evaporates due to the heat, so that the portion of contact between the lid and the container is sealed by water and the lid sticks to the container because of reduced pressure caused when heat escapes through the lid, thereby making it difficult to remove the lid from the container when the food is to be eaten. Additionally, conventional lids do not attach very securely, causing the performance to be further reduced even if offset only slightly, and risking spillage of the contents during transport.
Additionally, when lids having double-walled structures are stacked, the handle portion of the bottom lid supports the bottom wall of the top lid, so that the stacking height is made higher by the amount of protrusion of the handle portions, thereby reducing the storage capabilities and stability.
Additionally, as an example of a conventional manufacturing method for thermally insulated double-walled synthetic resin containers, there is a method wherein a resin with a high gas-barrier function is used to form a double-walled container by blow-molding with a low thermal conductivity gas, and the low thermal conductivity gas is sealed inside the double-walled container. With this method, gas-barrier capabilities are gained by using only resins, so that it is difficult to form radiation blocking materials in the thermally-insulating space.
As an example of another manufacturing method, the inner and outer containers are formed separately by means of extrusion molding, after which the respective mouth portions are attached with adhesives or the like to form a double-walled container, then the air between the inner and outer containers is replaced with low thermal conductivity gas. The inner container and outer container formed by this method allow radiation blocking materials composed of plating or metallic foil to be formed on the outer surface of the inner container or the inner surface of the outer container. After these radiation blocking materials are formed, the inner container is put into the outer container and the respective mouth portions are attached to form the space portion. Then, air is exhausted from the space portion through a hole formed in either the inner or the outer container, after which the space portion is filled with low thermal conductivity gas and sealed with a sealing panel.
The resins with high gas-barrier properties which are commonly known are highly moisture-absorbent, and their gas-barrier capabilities, strength, and dimensional stability are reduced when they absorb moisture. Additionally, under high temperatures, they tend to have insufficient strength. With regard to double-walled containers formed by a blow-molding method, it is extremely difficult to form metallic foil or plating onto the wall surface facing the insulating layer.
Additionally, when the inner container and the outer container are formed by means of extrusion molding, the inner and outer containers are attached to form a double-walled container, after which the air is removed from the space portion through a hole formed in either of the inner and outer containers. Then, low thermal conductivity gas is supplied and the hole sealed to complete the thermally insulated double-walled container. Thus, there are a lot of manufacturing steps and the manufacturing costs are expensive. Furthermore, the attachment by means of adhesives is often lacking in hot-water-resistance and chemical-resistance, so that there is the risk of decreased strength as well.